Severe asthma afflicts about 10% of asthma patients, but utilizes 30 to 50% asthma-associated healthcare costs. It is imperative to understand the mechanisms by which severe asthma in particular acute exacerbation develops. To improve the understanding of severe asthma and to develop better treatments, for the last 14 years, NIH/NHLBI has funded the Severe Asthma Research Program (SARP), the world's most comprehensive study of adults and children with severe asthma that are currently performed at seven leading asthma research sites in the US. The current SARP (SARP3) has started collecting upper and lower airway samples to study severe asthma pathobiology. This offers an unprecedented opportunity to further our understanding of natural history of asthma, disease progression and new therapeutic interventions. However, SARP3 protocols do not include the collection of minimally invasive nasal airway epithelial cells, an approach that can be repeated longitudinally to study asthma pathogenesis. Thus, we propose a time-sensitive study by using nasal airway epithelial cells to determine the role of a novel host defense protein SPLUNC1 in severe asthma. Short palate, lung, and nasal epithelium clone 1 (SPLUNC1) is a secretory protein from large airway (nasal, tracheal and bronchial) epithelial cells and is abundant in the epithelial lining fluid of healthy individuals. Through collaboration with SARP investigators, we identified SPLUNC1 as one of the most significantly decreased proteins in bronchoalveolar lavage fluid samples from severe asthmatics. We have found that SPLUNC1 exerts host defense functions against human rhinoviruses and bacteria involved in asthma exacerbations - a leading cause of morbidity, mortality and health care costs in asthma. We discovered SPLUNC1-deficient mice have exaggerated airway allergic inflammation following allergen challenges (e.g., eosinophils and mucus, relevant to asthma exacerbations). Moreover, transgenic over-expression of human SPLUNC1 in mouse airways attenuates allergen-induced eosinophilic inflammation. However, the role of SPLUNC1 in human severe asthma is unclear. We hypothesize that down-regulation of SPLUNC1 in asthmatic airways increases rhinovirus infection and associated asthma exacerbations contributing to asthma severity. In continuing collaboration with SARP and its data coordination center, we will define the role of SPLUNC1 in severe asthma phenotypes. Furthermore, we will team up with 4 SARP sites to collect brushed live nasal airway epithelial cells from asthmatic children and adults during asthma exacerbations to determine if lower baseline SPLUNC1 levels predispose the patients to rhinovirus infection and asthma exacerbations. Finally, we will mechanistically determine SPLUNC1's anti-viral and anti-inflammatory functions that are critical to the prevention of asthma exacerbations. Successful completion of this time-sensitive ancillary R01 proposal will guide use of SPLUNC1 as a biomarker and a new therapy to prevent or treat acute asthma exacerbations.